Free Astronomy Magazine March-April 2018

41 MARCH-APRIL 2018 EXOPLANETS T he three illuminated domes of ExTrA in the fore- ground, with numerous other structures belong- ing to the Chilean observatory La Silla in the background. [ESO/Emmanuela Rimbaud] does not add to that of the star, as happens in all the out-of-transit orbital positions. Using the instruments currently available to researchers, the only configuration that, in the most favourable cases, can analyse a planet’s atmosphere is its transit across the star’s disc. This is because of the high sig- nal-to-noise ratio (S/N) required for the spectroscopic examination. Per unit of ob- serving time and for a given planetary size,